The present invention is directed to a catheter having an enhanced ablation tip electrode.
Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity.
In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral artery, and then guided into the chamber of the heart which is of concern. Within the heart, the ability to control the exact position and orientation of the catheter tip is critical and largely determines how useful the catheter is.
A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue which is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, possibly above 60xc2x0 C., a thin transparent coating of dehydrated blood protein can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer can become progressively thicker resulting in blood coagulation on the electrode surface. Because dehydrated biological material has a higher electrical resistance than endocardial tissue, impedance to the flow of electrical energy into the tissue also increases. If the impedance increases sufficiently, an impedance rise occurs and the catheter must be removed from the body and the tip electrode cleaned.
In clinical practice, it is desirable to reduce or eliminate impedance rises and, for certain cardiac arrhythmias, to create larger and/or deeper lesions. One method for accomplishing this is to monitor the temperature of the ablation electrode and to control the RF current delivered to the ablation electrode based on this temperature. If the temperature rises above a preselected value, the current is reduced until the temperature drops below this value. This method has reduced the number of impedance rises during cardiac ablations but has not significantly increased lesion dimensions. The results are not significantly different because this method still relies on the cooling effect of the blood which is dependent on location in the heart and orientation of the catheter to endocardial surface.
Another method is to irrigate the ablation electrode, e.g., with physiologic saline at room temperature, to actively cool the ablation electrode instead of relying on the more passive physiological cooling of the blood. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm. However, irrigated electrodes, which require a fluid delivery pump, are generally more expensive than standard ablation catheters. Additionally, the saline or other fluid used in the irrigated electrodes tends to accumulate in the patient.
Accordingly, a need exists for an improved approach for creating deeper ablation lesions. SUMMARY OF THE INVENTION
The present invention is directed to a catheter having an enhanced tip electrode that permits the creation of deeper lesions. In one embodiment, the invention is directed to an ablation catheter comprising an elongated, flexible catheter body having proximal and distal ends and at least one lumen extending therethrough. A tip electrode having a length of at least about 3 mm is mounted on the distal end of the catheter body. The tip electrode comprises a base material having an outer surface and a porous layer applied over at least a portion of the outer surface of the base material, the porous layer comprising metal nitride, metal oxide, metal carbide, metal carbonitride, carbon, carboxy nitride, or a combination thereof. Preferably a temperature sensor is mounted in the tip electrode.
In another embodiment, the invention is directed to an ablation system comprising a catheter and a source of radio frequency energy. The catheter comprises an elongated, flexible catheter body having proximal and distal ends and at least one lumen extending therethrough. A tip electrode is mounted on the distal end of the catheter body. The tip electrode comprises a base material having an outer surface and a porous layer applied over at least a portion of the outer surface of the base material, the porous layer comprising metal nitride, metal oxide, metal carbide, metal carbonitride, carbon, carboxy nitride, or a combination thereof. The source of radio frequency energy is electrically connected to the tip electrode.
In another embodiment, the invention is directed to a method for ablating tissue in a patient. The method comprises providing a catheter as described above. The distal end of the catheter is introduced into the patient so that the tip electrode is in contact with the tissue to be ablated. Energy is applied to tip electrode, thereby creating a lesion in the tissue.